An integrated circuit (IC) package generally includes, among others, a substrate and a die placed on the substrate. The die is generally coupled to the substrate through wire bonds or solder bumps. Signals from the integrated circuit die may then travel through the wires or solder bumps to external circuitry through the substrate.
A package substrate is typically formed using low cost organic buildup material such as the ABF-GX13 insulation film material available from Ajinomoto. Transmission line structures such as striplines are commonly formed in the package substrate to provide good isolation between adjacent high-speed channels (i.e., channels transmitting at greater than 10 Gbps). In particular, the stripline is uniformly filled with the buildup material (i.e., each surface of a stripline signal conductor is in direct contact with the buildup material).
Buildup material such as ABF-GX13, however, exhibits undesirable properties when supporting high-speed data transmission. For example, the ABF-GX13 material is fairly lossy (i.e., energy associated with transmitted signals is likely to be absorbed by this lossy material) and exhibits a dielectric constant that varies with frequency (i.e., ABF-GX13 is dispersive). As a result, data conveyed through such types of striplines tend to exhibit high intersymbol interference (IS) jitter, which limits the trace length of striplines that are used for high-speed data transmission.